With the phenomenal growth of the internet of things (IoT) devices in 5G scenarios (including smart city, connected health, industrial internet, vehicle network), a huge number of wireless-enabled sensors are broadly deployed. A great deal of sensing data is collected sporadically, and then forwarded to the cloud via connected IoT devices, cellular networks and internet [1]}, [2]}. It has been predicted in Ericsson's report that the number of short-range and cellular IoT devices will reach 17.8 billion and 4.1 billion by 2024, respectively [3]}. Moreover, the visual networking index (VNI) released by Cisco has forecasted that machine to machine (M2M) connections in IoT networks will grow from 6.1 billion in 2017 to 14.6 billion by 2022 [4]}. The proliferation of IoT devices will pose the necessity of massive wireless connections to confront the ongoing paradigm of 5G and beyond. In addition, the non-orthogonal multiple access (NOMA) has been recognized as a promising technology enabler for IoT to offer massive connectivity over limited wireless resources [5]}, [6]}, [7]}, [8]}. The essence of NOMA is to serve multiple users over the same frequency/time/code resource by means of superposition coding and successive interference cancellation (SIC). In particular, NOMA is capable of attaining an improved spectral efficiency over its conventional counterpart, orthogonal multiple access (OMA). In addition, NOMA can also strike a balanced tradeoff between the system throughput and users' fairness. All these intrinsic characteristics of NOMA are beneficial to the development and deployment of IoT. Moreover, in order to get rid of the high complexity and severe error propagation of SIC, the combination of NOMA and OMA technologies has been recognized as a feasible solution for resource-limited IoT networks [9]}, [7]}. Particularly, the hybrid multiple access (MA) scheme first groups users into pairs to perform NOMA, then multiplexes different pairs over orthogonal resources.
